In carrier telecommunication, for example, it is desirable to measure the power level of incoming signals over an entire base-band as it appears in its transposed frequency position, e.g. in a range of 60-180 kHz or of 312-552 kHz as conventionally used for 12-channel or 60-channel voice transmission. Systems are known, e.g. from commonly owned U.S. Pat. Nos. 3,461,385, 3,480,862 and 3,486,112, which allow the exploration of the signal level in any desired narrow-band section of such a wide frequency range by heterodyning the incoming test signals with a modulating frequency so chosen that the resulting beat-frequency oscillations clear an intermediate-frequency filter; by periodically shifting or "wobbling" the modulating frequency between suitable limits, continuously or in steps, information can be gained on the magnitude of the signal level throughout the range of interest (see brochures on Selective Level Meters SPM-6 and SPM-60 of Wandel u. Goltermann, the assignee of my present application). Such information, however, does not provide a numerical value for the entire range which could be compared with a standard and which would enable an evaluation of the performance of the overall transmission system under different operating conditions. Currently available wide-band level meters are generally limited to bandwidths not exceeding 48 kHz, owing to the imperfections of intermediate-frequency filters with large pass bands; they are, moreover, restricted to specific frequency ranges whose spread could be narrowed (but not widened), in a rather cumbersome manner, only with the aid of additional filters in cascade therewith.